I'm not too sure what's around that area, but South of Grants pass is the Mcgrew trail. There's a few youtubes out there and this link has some info on it.
For the dunes you just need a flag whip and a Oregon ATV sticker. Pretty much any motorcycle shop has them both. The biggest area starts down by Coos Bay and runs north to just south Of Winchester Bay. Winchester has the tallest dunes, but not much fun for a jeep. Florence is also a good spot. Some taller dunes and a few trails to play on.
2012 Rubicon JKUR: 4" Tera Flex Long Arm, Fox 2.0 Shocks w/resi, Fox Stabi, Yukon 5:13's, Evo C gussets, Evo Magnum Sleeves, Evo Rockstars, LOD Stubby Bumper, Poison Spyder Rock Brawler Rear Bumper/Carrier,Poison Spyder Sliders, Poison Spyder EVAP Skid, KMC Strikes on 37" Nitto Trail Grapplers